1. Field of the Invention
The present invention relates to a lock-up clutch mechanism used with a torque converter of an automatic transmission for a vehicle, and more particularly, it relates to an improvement in a friction sliding surface of the lock-up clutch mechanism.
2. Related Background Art
In torque converters used in automatic transmissions, although smooth starting, acceleration and deceleration can be achieved, since a power is transmitted via fluid, a transmitting efficiency is worsened. Thus, there has been proposed a technique in which, if a speed of the vehicle exceeds a predetermined value, in order to reduce energy loss and to enhance reduction of fuel consumption, a lock-up clutch mechanism having a lock-up clutch is operated to directly connect an engine to drive wheels.
Further, in recent years, in order to further enhance the reduction of the fuel consumption, the lock-up clutch mechanism has been operated even when the vehicle is being driven at a low speed. In this case, in order to reduce vibration of the engine at the low speed and transmission shock, a so-called slip lock-up control for performing a lock-up control while maintaining a slip amount at predetermined revolutions per minute is adopted.
In general, in the lock-up clutch, self-exciting vibration referred to as judder may be generated, thereby worsening comfort of the vehicle considerably. The judder is greatly influenced by imbalance of face pressure distribution on a friction surface in a circumferential direction during the slip. The imbalance of the face pressure distribution is greatly influenced by accuracy of the friction surface, and, thus, there arises dispersion in the circumferential direction of the friction surface by slight undulation of a piston of the lock-up clutch and/or strain of attaching bolts for a drive plate provided on a front cover.
In order to suppress such judder, for example, in Japanese Patent No. 3157213, a friction surface to be engaged at the initiation of engagement is formed as a cut low resin ratio layer.
Further, Japanese Patent Application Laid-Open No. 2004-11710 discloses a lock-up clutch having a friction engaging surface comprising a tapered surface in which a thickness of a friction material is reduced from a radial outer periphery to a radial inner periphery.
However, since the generation of the judder is greatly based on the imbalance of the face pressure distribution on the friction surface in the circumferential direction during the slipping, in the techniques disclosed in the Japanese Patent No. 3157213 and the Japanese Patent Application Laid-Open No. 2004-11710, the judder could not necessarily be suppressed satisfactorily. Further, it was difficult to obtain the greater torque transmitting capacity.
Further, in a condition that maximum engaging pressure is generated during the engagement of the lock-up clutch, the inner peripheral portion of the friction material of the lock-up piston is contacted with the front cover. In this case, the entire surface of the lining to be contacted remains a configuration obtained when it is stuck or adhered, and thus, there is roughness or undulation on the surface of the friction material. From this condition, if the friction material is engaged with or tightened to an opponent member, there is a danger of generating local areas where a void is created between the friction material and the opponent member and local areas where higher contact pressure is created between the friction material and the opponent member.
As a result, a contacting condition at the interface between the friction material and the opponent member becomes unstable to generate the judder easily, and a sealing ability of the interface is worsened, thereby reducing the capacity.